Recently, a digital satellite broadcasting system becomes widespread increasingly. This system may transmit a bit stream obtained when video and audio signals are digitally compression-coded according to the MPEG standard or the like and multiplexed according to the MPEG standard or the like through a satellite. The receiving side may receive such bit steam, separate video and audio data from the received bit stream and decode the resultant bit stream to provide video and audio signals.
As the bit stream, there may be used an MPEG2 (Moving Picture Experts Group 2) transport stream. FIG. 15B shows the MPEG2 transport stream which may comprise a series of 188-byte fixed length transport stream packets (hereinafter referred to as “TS packets”) of #1 to #3 programs. Each TS packet may comprise a 4 byte packet header and a 184 byte adaptation field and/or payload as shown in FIG. 15A.
The packet header may comprise a sync byte for detecting the leading portion of the TS packet, PID (Packet Identification: packet identification data) indicative of an attribute of individual stream (data string) of the corresponding packet and adaptation field control information indicating whether this packet has the adaptation field and whether this packet has the payload, or the like. The adaptation field may include added information concerning individual stream and stuffing bytes (invalid data bytes). The payload may comprise video and audio PESs (Packetized Elementary Stream) which are re-divided.
Heretofore, there had been put into practical use the technologies capable of improving a picture quality and a transmission efficiency by processing inputted data at a variable rate. A data multiplexing apparatus is able to multiplex a plurality of variable-rate inputted data by using a technology such as statistical multiplexing. However, because inputted data is processed at the variable rate, there arise various problems such that a delay in the multiplexing is increased by complexity of control and that excessive restrictions are imposed upon the encoder side.
FIG. 16 shows an example of the manner in which a total sum of inputted rates is fluctuated when inputted data concerning video signals Va, Vb, Vc are multiplexed. During a time period ranging from a time t1 to a time t2, the total sum of inputted rates exceeds an output transmission rate R1 so that data obtained in that time period cannot be transmitted or can be transmitted after having delayed within a buffer. When data are allowed to be delayed within the buffer, if the transmission rate is of the fixed transmission rate, then a quantity in which data are stored in the buffer will increase or decrease depending upon a degree in which the input rate can be varied. As a result, in some cases, a delay time required when inputted data are multiplexed increases considerably to cause a synchronization to be failed on the receiving side, which then leads to the interruption of an image and sounds. Therefore, the transmitting side should prevent such accident.
Moreover, when inputted data having priority information are multiplexed, in general, inputted data with a high priority may be multiplexed with a priority and other inputted data are stored in the buffer during that time period. Therefore, in this case, the fluctuation of the quantity in which data are stored in the buffer may become more remarkable.
It is an object of this invention to provide a data multiplexing apparatus or the like in which the increase of the delay time caused when inputted data are multiplexed can be avoided to prevent a disadvantage such as a failure of synchronization on the receiving side.